Preparation of carboxylic acids by carbonylation of alcohols

ABSTRACT

Carboxylic acids having the structural formula R 1  COOH, wherein R 1  is a linear, branched or cyclic chain alkyl radical of from 1 to 6 carbon atoms or a phenyl-C n  H 2n  -radical wherein 1≦n≦6, are prepared by carbonylating an alcohol having the formula R 1  -OH with carbon monoxide in liquid phase, in the presence of a catalytically effective amount of nickel and an alkyl or acyl halide promoter therefor, at a temperature of at least about 120° C. and under a total pressure of less than 200 bars, and said carbonylation being carried out in the presence of at least one lanthanide salt, and in an initial carboxylic acid reaction medium having the formula R 2  -COOH, wherein R 2  is defined as was R 1  above, and further wherein R 1  and R 2  may be the same or different.

CROSS-REFERENCE TO RELATED APPLICATIONS

Our copending applications, Ser. No. 235,742 filed Feb. 18, 1981; Ser.No. 247,744, filed Mar. 26, 1981; and Ser. No. 260,810, filedconcurrently herewith, all assigned to the assignee hereof, and allhereby expressly incorporated by reference and relied upon.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the preparation of carboxylic acids,and, more especially, to the preparation of carboxylic acids, notablyacetic acid, by carbonylation of an alcohol.

2. Description of the Prior Art

It is well known to this art that, e.g., acetic acid, can be prepared bycarbonylation of methanol under relatively severe reaction conditions ofpressure, in the presence of nickel and a free or bound halogen.

Thus, it has been proposed, in particular (compare U.S. Pat. No.2,729,651), to carry out the carbonylation of methanol in the presenceof nickel complexes which are obtained by reacting nickel halides withquaternary ammonium (or phosphonium) halides, i.e., complexes of thegeneral formula:

    [A.sub.4 M].sub.2 NiX.sub.4

in which X represents a bromine or iodine atom, M represents aphosphorus or nitrogen atom and A is, for example, a lower alkylradical.

These complexes can be used in the aforesaid form in the reaction inquestion, or they can be formed in situ. However, although increasedpressure (on the order of 700 atmospheres) is applied during thecarbonylation reaction, the efficiency of the catalyst system, expressedin terms of hourly output or yield, is very low.

It has been possible to substantially improve this output, expressedeither with respect to the reaction volume or with respect to the amountof nickel used, by employing in the subject reaction a nickel halide, onthe one hand, and, on the other hand, a quaternary ammonium (orphosphonium) halide in a quantity greater than that required by thestoichiometry of formation of the complexes of the aforenoted formula.Compare German Pat. No. 933,148. However, in this latter case, thepressure conditions remain severe.

More recently, catalyst systems which permit the carbonylation ofmethanol under less severe conditions of pressure have been proposed.Thus, published French Pat. application No. 2,370,023 describes thecarbonylation of methanol in the presence of at least 10 mols of methyliodide per 100 moles of methanol, and in the presence of nickel and of afree phosphine and/or a phosphine complexed with the nickel, under apressure of less than 70 bars. However, the efficiency of such a system,again expressed in terms of hourly output, remains low.

In the context of a carbonylation process under 1ow pressure, it hasalso been emphasized (compare published French Pat. application No.2,404,618) that the presence of solvents, such as carboxylic acids ortheir esters, has a favorable effect on the course of the carbonylationreaction. However, utilization of such solvents is not essential, andthe starting material alcohol can also serve as the solvent.

However, the industrial or commercial value of these recent developmentsis limited as a result of the instability and the cost of the phosphinesor of the amines required to effect these techniques.

SUMMARY OF THE INVENTION

Accordingly, a major object of the present invention is the provision ofan improved process for the preparation of carboxylic acids, notablyalkanoic acids, and especially acetic acid, such improved process beingcharacterized by markedly enhanced output, by carbonylation of alcoholsin the presence of nickel and at least one halogen-containing promoterunder reaction conditions of relatively mild pressure, and such processbeing conspicuously devoid of the aforenoted disadvantages anddrawbacks.

Briefly, the present invention features an improved process for thecarbonylation of alcohols in liquid phase, under a total pressure ofless than 200 bars, in the presence of a catalytically effective amountof nickel, an alkyl or acyl halide and at least one lanthanide salt, anda beginning amount of a carboxylic acid.

DETAILED DESCRIPTION OF THE INVENTION

More particularly according to this invention, it has now surprisinglybeen found that, e.g., the carbonylation of methanol in acetic acidproceeds quite satisfactorily when the partial pressure of the carbonmonoxide is maintained relatively low; this is all the more unexpectedsince the state of this art has to date clearly pointed to the necessityfor carrying out the carbonylation under very high pressure.

Even more particularly, the focus of the present invention is thedemonstrated surprising activity of the lanthanide salts, when used asco-catalysts, in the carbonylation of an alcohol, in liquid phase, undera total pressure of less than 200 bars. Indeed, it has even beenconfirmed that the salts and other derivatives of numerous metals(including metal carbonyls) do not permit the reaction of carbonmonoxide with an a1cohol in a carboxylic acid medium under the aforesaidreaction conditions, in the presence of nickel and a halogen-containingpromoter.

According to the present invention, carbon monoxide is reacted with analcohol in order to produce the corresponding carboxylic acid, inaccordance with the following equation: ##STR1## in which R¹ representsa linear, branched or cyclic alkyl radical containing from 1 to 6 carbonatoms, or a radical phenyl-C_(n) H_(2n) --, in which n is an integerranging from 1 to 6 (1≦n≦6). The radical R¹ can also bear one or moresubstituents which are inert under the reaction conditions of thepresent invention. R¹ is preferably a linear or branched chain loweralkyl radical having from 1 to 4 carbon atoms, and more preferably is amethyl radical.

The process according to the present invention is carried out in theliquid phase in a reaction medium comprising a carboxylic acid of theformula R² COOH, in which R² is defined exactly as was R¹, it thus beingpossible for R² and R¹ to either be the same or different. Expressedotherwise, the carboxylic acid which, so to speak, functions as asolvent is not necessarily the particular carboxylic acid produced bythe carbonylation reaction. However, it may indeed prove preferable forthe carboxylic acid employed as the solvent to be that produced in thereaction. Needless to say, employing as the solvent a carboxylic acidwhich is heavier than the acid produced can facilitate the separationoperation.

The carboxylic acid R² COOH advantageously constitutes at least 10% byvolume of the initial reaction mixture. It preferably constitutes atleast 20% by volume of said reaction mixture. It can constitute asubstantial proportion of the reaction mixture, especially in the caseof an operation carried out continuously, by injecting the alcohol R¹ OHinto the carbonylation reactor.

The process according to the invention requires the presence of acatalytically effective amount of nickel. Any source of nickel can beused within the scope of the present process. Thus, it is possible tointroduce the nickel in its metallic form (for example Raney nickel) orin any other convenient form. Examples which are exemplary of nickelcompounds which can be utilized for carrying out the present processare: the carbonate, oxide, hydroxide, halides, in particular the iodide,and carboxylates, in particular the acetate, or nickel. Nickel carbonylis also particularly suitable. Raney nickel, nickel iodide, nickelacetate and nickel carbonyl are preferred to be used.

The amount of nickel is not critical. The proportion of nickel whichaffects the rate of reaction is determined as a function of the rate ofreaction calculated as suitable. In general, an amount of nickel between5 and 2,000 milligram-atoms per liter of solution gives satisfactoryresults. The reaction is preferably carried out with an amount of nickelbetween 20 and 1,000 milligram-atoms per liter.

The process according to the present invention a1so requires thepresence of at least one alkyl or acyl halide. The formulae of thesehalides are, respectively, ##STR2## in which X represents a chlorine orbromine atom, or, preferably, an iodine atom and R³ is defined exactlyas was R¹ (and R²), it being possible for R³ and R¹ (or R³ and R²) to beidentical or different. The alkyl halide which can initially be used inthe reaction mixture can, of course, be formed in situ, starting fromhalogen derivatives, such as Cl₂, Br₂, I₂, HCl, HBr, HI, NiBr₂ and NiI₂,and from the alcohol (starting material). Stated differently, some orall of the alkyl halide required for carrying out the present processcan be formed starting from its "precursors" defined above.

Furthermore, it will be noted that when the halogen derivative isselected from among the nickel compounds, it can be considered not onlyas a precursor of the alkyl halide but also as a precursor of themetallic catalyst. In this particular case, it is also preferred to add,initially, an alkyl or acyl halide and/or a precursor other than thenickel halides in question.

Within the ambit of the present invention, lower alkyl iodides havingfrom 1 to 4 carbon atoms define a preferred class of alkyl halides.Methyl iodide is particularly suitable for carrying out the processaccording to the invention.

An alkyl or acyl halide concentration of at least 0.5 mol per liter ofreaction mixture is typically required for successfully carrying out thepresent process. Although increasing the concentration of alkyl or acylhalide has a favorable effect on the rate of reaction, it is preferrednot to exceed a concentration on the order of 8 mols per liter. For thisreason, a concentration of alkyl or acyl halide between 0.8 and 6mols/liter, and preferably between 1.5 to 5 mols/liter providessatisfactory results.

One of the essential characteristics of the present invention is the useof at least one lanthanide salt co-catalyst, preferably at least onecompound selected from the group comprising the halides, the hydroxides,the oxychlorides, the carbonates, the oxalates and the carboxylates ofthose elements having an atomic number ranging from 57 to 71, inclusive;such carboxylate anions can be represented by the formula R⁴ -COO⁻, inwhich R⁴ is defined exactly as was R¹ and it being possible for R⁴ andR¹ to be either the same or different; the 1anthanide salts can also bein hydrated form. The salts of lanthanum, cerium and neodymium areespecially preferred for carrying out the process according to thisinvention.

The precise nature of the anionic moiety of the subject lanthanide saltsdoes not appear to be a fundamental parameter of the present process.The lanthanide carboxylates, and more particularly the acetates, areconvenient to use and in this respect are recommended for carrying outthe present invention. The acetates of lanthanum, cerium and neodymiumare notably suitable.

Good results are obtained if the atomic ratio M/Ni, M denoting alanthanide, is between 0.1 and 200, although lower or higher ratios canbe selected. This ratio is advantageously a value ranging from 0.5 to100 and preferably ranging from 1 to 50.

A reaction temperature of at least 120° C. is typically required inorder to obtain a satisfactory rate of reaction. A temperature range offrom 160° to 220° C. proves advantageous.

According to the present invention, it is not necessary to purify or drythe alcohol and the carboxylic acid initially used in the reaction.Technical grade alcohols and carboxylic acids optionally containing upto 20% by volume of water can be utilized. On the other hand, some orall of the alcohol R¹ OH can be employed in the reaction in the form ofan ester R² COOR¹, R¹ and R² being as defined above and it beingpossible for R¹ and R² to be identical or different. In this case, anamount of water at least equal to the amount theoretically required tohydrolyze the ester initially introduced should also initially be used.

The carbonylation process according to the present invention is carriedout in the liquid phase under a pressure greater than atmosphericpressure, the pressure being, however, less than 200 bars. Moreparticularly, it is recommended to carry out the reaction under apartial pressure of carbon monoxide of between 10 and 100 bars. Thecarbon monoxide is preferably employed in an essentially pure form, suchas is commercially available. However, the presence of impurities, suchas, for example, carbon dioxide, oxygen, methane and nitrogen, is notharmful. The presence of hydrogen, even in relatively large proportions,likewise is not harmful.

Upon completion of the reaction, the reaction mixture is separated intoit various constituents by any appropriate means, for example, bydistillation.

The process according to the invention is particularly suitable for thepreparation of acetic acid by carbonylation of methanol, especially inan acetic acid reaction medium.

In order to further illustrate the present invention and the advantagesthereof, the following specific examples are given, it being understoodthat same are intended only as illustrative and in nowise limitative.

The following symbols and definitions have been used in the saidexamples:

AcOH designates acetic acid;

AcOMe designates methyl acetate;

RY designates the molar ratio: ##EQU1## i.e., the molar ratio betweenthe "potential acetic acid" formed and the initial (CH₃ OH+CH₃ I);

t denotes the effective duration of the absorption of the carbonmonoxide at the temperature of the experiment;

T denotes the duration of the experiment at the specified temperature;and

Pr denotes the productivity or outout, relative to the time t (expressedin hours), in grams of "potential acetic acid" formed per liter of theinitial reaction mixture.

EXAMPLE 1

The following ingredients were introduced into a Hastelloy B 2 autoclaveof 125 ml capacity:

(i) 372 mmols of methanol, i.e. 15 ml;

(ii) 360 mmols of acetic acid, i.e., 20 ml;

(iii) 202 mmols of methyl iodide, i.e. 12.5 ml;

(iv) 40 mg atoms of lanthanum in the form of lanthanum acetate, i.e.,12.64 g; and

(v) 20 mmols of nickel acetate tetrahydrate, i.e. 5.0 g.

After closing the autoclave, a pressure of 40 bars of carbon monoxidewas established therein. Shaking by means of a reciprocating system wascommenced and the autoclave was heated to 180° C., over the course ofabout 20 minutes, by means of an annular furnace. The pressure in theautoclave was then 66 bars; it was subsequently maintained at 70 bars byintroduction of additional amounts of pure CO.

The absorption of carbon monoxide was complete after a reaction time of2 hours at 180° C.; heating was nevertheless continued for an additional5 minutes at this temperature.

The shaking and heating were then stopped; the autoclave was cooled anddegassed.

After dilution, the resulting reaction mixture was analyzed by gaschromatography. Same contained 51.4 g of acetic acid (RY=87%) and 0.30 gof methyl acetate.

The productivity (Pr) of the reaction in terms of acetic acid wastherefore 300 grams per hour and per liter (g/hour×liter).

EXAMPLES 2 TO 6

Example 1 was repeated, using the same apparatus, but varying theco-catalysts. The particular conditions and also the results obtainedare reported in Table I below.

Control experiments (a) to (1):

Also using the equipment and the procedure described in Example 1, aseries of experiments was carried out on a charge comprising 15 ml ofmethanol, 20 ml of acetic acid and methyl iodide at 180° C. and under 70bars of pressure, in the presence of various metal compounds. Noreaction was observed after 2 hours at the temperature indicated.

The particular conditions of these control experiments are reported inTable II below.

                                      TABLE I                                     __________________________________________________________________________    NICKEL COMPOUND  CO-CATALYST                    Pr                            Example     mg         mg          AcOH                                                                              AcOMe                                                                              RY  g/hr.                         No.  type   atoms                                                                              type  atoms                                                                             T   t   (g) (g)  (%) ×/liter                 __________________________________________________________________________    1    Ni(OAc).sub.2                                                                        20   La(OAc).sub.3                                                                        40 2 hr.                                                                             2 hr.                                                                             51.4                                                                              0.30 87  300                                                      05 min.                                            2    "      "    Ce(OAc).sub.3                                                                       "   2 hr.                                                                             2 hr.                                                                             58.0                                                                              0    100 370                                                      07 min.                                            3    "      "    Nd(OAc).sub.3                                                                       "   1 hr.                                                                             1 hr.                                                                             54.7                                                                              0    97.5                                                                              350                                                      55 min.                                                                           55 min.                                        4    "      "    Eu(OAc).sub.3                                                                       "   2 hr.                                                                             2 hr.                                                                             47.6                                                                              0.81 70  240                                                      10 min.                                                                           10 min.                                        5    "      "    Yb(OAc).sub.3                                                                       "   3 hr.                                                                             2 hr.                                                                             52.6                                                                              0    91  230                                                          45 min.                                        6    Ni(CO).sub.4                                                                         "    Ce(OAc).sub.3                                                                       100 2 hr.                                                                             2 hr.                                                                             25.8                                                                              8.81 37  ND                            __________________________________________________________________________     NB: in this table, Ni(OAc).sub.2 denotes nickel acetate tetrahydrate.         ND: not determined.                                                      

                                      TABLE II                                    __________________________________________________________________________    Control experiments                                                                                CH.sub.3 I                                               Control NICKEL COMPOUND                                                                            in  METAL COMPOUND                                       Experiment No.                                                                        Type    mg atoms                                                                           mmols                                                                             Type     mg atoms                                    __________________________________________________________________________    a       Ni(CO).sub.4                                                                          20   201 Cr(CO).sub.6                                                                           50                                          b       Ni(CO).sub.4                                                                          20   206 Zn(OAc).sub.2.2H.sub.2 O                                                               100                                         c       Ni(CO).sub.4                                                                          20   203 Zn       50                                          d       Ni(CO).sub.4                                                                          20   209 Mn(OAc).sub.2.4H.sub.2 O                                                               100                                         e       Ni(CO).sub.4                                                                          20   204 AlI.sub.3                                                                              50                                          f       Ni(CO).sub.4                                                                          20   203 Co(OAc).sub.2.4H.sub.2 O                                                               100                                         g       Ni(OAc).sub.2.4H.sub.2 O                                                              10   205 MoBr.sub.3                                                                             30                                          h       Ni(OAc).sub.2.4H.sub.2 O                                                               8   202 Fe(OAc).sub.2                                                                          100                                         i       Ni(CO).sub.4                                                                          20   205 MoI.sub.3                                                                              10                                          j       Ni(CO).sub.4                                                                          10   209 WI.sub.3 10                                          k       Ni(CO).sub.4                                                                          20   211 Mg(OAc).sub.2.4H.sub.2 O                                                               100                                         l       Ni(CO).sub.4                                                                           8   200 BiI.sub.3                                                                              40                                          __________________________________________________________________________

The principles, preferred embodiments and modes of operation of thepresent invention have been described in the foregoing specification.The invention which is intended to be protected herein, however, is notto be construed as limited to the particular forms disclosed, sincethese are to be regarded as illustrative rather than restrictive.Variations and changes may be made by those skilled in the art withoutdeparting from the spirit of the invention.

What is claimed is:
 1. A process for the preparation of a carboxylicacid having the structural formula R¹ COOH, wherein R¹ is a linear,branched or cyclic chain alkyl radical of from 1 to 6 carbon atoms or aphenyl-C_(n) H_(2n) -radical wherein 1≦n≦6, comprising carbonylating analcohol having the formula R¹ -OH with carbon monoxide in liquid phase,in the presence of a catalytically effective amount of nickel and analkyl or acyl halide promoter therefor, at a temperature of at least120° C. and under a total pressure of less than 200 bars, saidcarbonylation being carried out in the presence of at least onelanthanide salt, and in an initial carboxylic acid reaction mediumhaving the formula R² -COOH, wherein R² is defined as was R¹ above, andfurther wherein R¹ and R² may be the same or different.
 2. The processas defined by claim 1, wherein the alkyl or acyl halide is an iodide. 3.The process as defined by claim 2, wherein the halide is a C₁ -C₄ -alkyliodide.
 4. The process as defined by claim 3, wherein the alkyl iodideis methyl iodide.
 5. The process as defined by any of claims 1 to 4,wherein R¹ is a C₁ -C₄ -alkyl radical and R² is a linear, branched orcyclic chain C₁ -C₆ -alkyl radical or a phenyl-C_(n) H_(2n) -radical, inwhich 1≦n≦6.
 6. The process as defined by claim 5, wherein R¹ is methyl.7. The process as defined by claim 1, wherein the carboxylic acid R²COOH comprises at least 10% by volume of the initial reaction medium. 8.The process as defined by claim 7, wherein the concentration of nickelin the reaction medium ranges from 5 to 2,000 mg-atoms per liter.
 9. Theprocess as defined by claim 8, wherein said concentration of nickelranges from 20 to 1,000 mg-atoms per liter of reaction medium.
 10. Theprocess as defined by claim 8, wherein the concentration of alkyl oracyl halide in the reaction medium ranges from 0.5 to 8 mols per liter.11. The process as defined by claim 10, wherein said concentration ofalkyl or acyl halide ranges from 0.8 to 6 mols per liter.
 12. Theprocess as defined by claim 1 or 10, wherein R¹ and R² are identical.13. The process as defined by claim 1 or 10, wherein the lanthanide saltis a halide, hydroxide, oxychloride, carbonate, oxalate or carboxylateof an element having an atomic number ranging from 57 to 71, the anionof the carboxylate having the formula R⁴ -COO⁻, in which R⁴ is definedas was R¹, and R⁴ and R¹ may be the same or different.
 14. The processas defined by claim 13, wherein the cation of the lanthanide salt islanthanum, cerium or neodymium.
 15. The process as defined by claim 14,wherein the anion of the lanthanide salt is the carboxylate having theformula R⁴ -COO⁻.
 16. The process as defined bv claim 15, said anionbeing the acetate.
 17. The process as defined by claim 13, wherein theatomic ratio M/Ni, with M being a lanthanide, ranges from 0.1 to 200.18. The process as defined by claim 17, wherein the ratio M/Ni rangesfrom 0.5 to
 100. 19. The process as defined by claim 18, wherein theratio M/Ni ranges from 1 to
 50. 20. The process as defined by claim 1,wherein the reaction temperature ranges from 160° to 220° C.
 21. Theprocess as defined by claim 20, wherein the partial pressure of thecarbon monoxide ranges from 10 to 100 bars.